This invention relates to nematic liquid crystal compositions of the type used for display and information handling and particularly relates to liquid crystal compositions suitable for use in field-effect (or "twisted nematic") liquid crystal display devices.
References No. 1 through 14, listed at the end of the specification, provide a background of the general subject matter of the present patent and References No. 15 through 25 relate more particularly to the compounds discussed herein. The content and disclosure of these references is incorporated herein by reference and a knowledge of the disclosures of these references is assumed. The operation and construction of field-effect or twisted nematic liquid crystal display units of the type referred to hereinafter are described in these references.
Twisted-nematic liquid crystal display devices conventionally consist of two transparent substrates spaced a very small distance, e.g. 12.5 microns apart, the space being filled with a "nematic liquid crystal". (Hereinafter a "nematic liquid crystal" will be understood as a material which assumes its "ordered" nematic liquid phase at the temperature assumed; and similarly for a "smectic liquid crystal material.) Upon the application of a voltage between the two substrates, the light transmissive characteristics of the liquid crystal can be controlled to provide light transmission or to block off the transmission of light. Thus, these liquid crystal devices can serve effectively as very rapidly acting "light gates". The prior art devices generally operate with a voltage of from about 6 volts to 10 volts or higher for cells in which the substrates are separated on the average by 12.5 microns. In practice, separations less than about 12.5 microns average are difficult to obtain on a repeatable basis and mass production of cells with smaller spacings is not practical.
The output of the most desirable (integrated) timing, counting, and like circuits for current liquid crystal devices is from about 2.5 to about 3.3 volts; however, in the prior art, it has been necessary to multiply this voltage two or more times in order to obtain sufficient voltage to actuate or start up liquid crystal devices.
The prior art has been faced with another problem in the high consumption of energy in integrated circuitry, voltage multipliers and the display of liquid crystal devices. Energy consumption is particularly important in certain portable display devices such as liquid crystal display wrist watches. It is a practical necessity that batteries last for six to twelve months in wrist watches. This has not heretofore been a readily achievable goal principally because the problem of power consumption continues to plague the industry. One method by which power consumption can be reduced is to operate the liquid crystal display device at the inherent output voltage (e.g. 2.5 to 3.3 volts), of a typical integrated circuit. This requires less power and with the elimination of the voltage multiplier circuitry, has offered a particularly advantageous route to the reduction of energy consumption. Heretofore, however, this source of reduced energy consumption has not been successfully pursued because of the impossibility, or the impracticability, of operating liquid crystal display devices having less than about five or six volts.
Biphenyl liquid crystals are taught in the prior art and considerable work has been done by Gray (Reference 19) and his co-workers. Nematic biphenyl liquid crystals have been shown to be considerably more stable than Schiff-base compounds and have been actuated with lower saturation voltages than Schiff base. Gray et al. (Reference 19) report, for example, that for a particularly attractive member of this class of compounds, 4'-n-pentyl-4-cyanobiphenyl (PCBP) the V.sub.th was 1.1 V.sub.rms and at an applied voltage of 3 V.sub.rms the compound gave acceptable decay and rise times of 150 and 100 milliseconds.
However, lack of general availability of high quality nematic biphenyl liquid crystal compounds, the extremely high cost of such compounds, and the narrow nematic temperature ranges exhibited by many of these class of compounds suggested the desirability of alternative compositions which would retain the advantages of nematic biphenyl and obviate the principal disadvantages of these materials.
I have therefore discovered that adding certain mixtures of ester-type liquid crystals to nematic biphenyl which retain to a large extent the advantage of nematic biphenyls (e.g. chemical stability and fast response times) but reduce the cost, reduce the electrical capacitance, and expand the operational temperature range of display devices which are actuated with low voltages (i.e. 3.5 V.sub.rms or less).
It had been assumed by those in the art that when two different types of nematic liquid crystals were mixed together, a composite nematic liquid crystal mixture would always result; for instance, that if a Schiff-base nematic was mixed with an ester-type nematic, a liquid crystal having nematic characteristics would be formed. However, I discovered that when certain ester-types are mixed with certain nematic biphenyls (i.e. cyanobiphenyls), an incompatible state results. Specifically, certain of these materials formed an undesirable smectic phase instead of a nematic phase. Moreover, some blends were not smectic but produced undesirable homeotropic alignment, especially in the "fill-hole" areas of the display.
I further discovered that this incompatible state (e.g. smectic phase) could be supressed by the addition of a third component which was "compatible" with the nematic biphenyl, i.e. a component which formed a satisfactory nematic phase when mixed with the biphenyl. Moreover, I discovered that such three-component mixtures were superior to a nematic biphenyl by itself as well as to two-component mixtures of nematic biphenyls and "compatible" liquid crystals. (See application Ser. No. 502,659 mentioned above.)
While these three-component blends also show much improved electro-optical properties over prior art liquid crystal compositions, some troublesome problems still persisted; for instance, extreme sensitivities in response times resulting from slight changes in positive dielectric anisotropy (PDA) concentration and external factors (e.g. different display glasses, dirt and impurities) have arisen.
But, I have also discovered that by designing the total composition in terms of PDA/NDA constituents and by the addition of a small amount of certain chiral additives, the latter problems will be overcome and, surprisingly, the mixtures will derive unexpected beneficial characteristics (as explained below). As a result of this discovery, blends made according to this invention can be optimized in terms of their electro-optical performance and so as to be less sensitive to minute changes in concentration and other factors.